A person offering a data storage service, such as a Storage Service Provider (“SSP”) or an information services department within a company, needs to ensure that performance requirements are met for accessing the stored data. It is common in computer systems for a single data storage system to be used to hold data for multiple storage clients, which may be different computers, different applications, or different users. When the data storage system is owned by a Storage Service Provider, different clients using the same system may be separate customers, with separate contractual arrangements with the SSP.
A storage system has many components that participate in the servicing of requests from clients. These include but are not limited to: arm actuators, data channels, disk controllers, memory buses, and protocol chips on the disk drives themselves; processors, memory controllers, buses, and protocol chips on storage system controllers; and SCSI buses, network links, loops, fabric switches, and other components for the client-to-controller and controller-to-disk interconnect. A request generally requires several of these components to participate at particular steps in its processing. Many components can generally be used concurrently, so that steps in the servicing of many requests are being performed simultaneously.
To facilitate the concurrent utilization of resources, the system is built with an ability to enqueue requests and the subtasks involved in servicing them. There is a tradeoff between throughput (the total number of requests or number of bytes processed) and response time (the elapsed time from when the request is received by the system and when its completion is reported to the client). To achieve maximum throughput, a client usually submits a large number of requests for data. The large request load enables efficient workload scheduling in the system, but the response time in this case may be many times greater than that for a lightly loaded system because the requests spend a long time in the queue before being serviced.
Typically, the storage system contains one or more storage devices such as disk drives for storing data in a persistent way. It also contains one or more processors that handle requests for access, generally calling upon the storage devices to do so. Associated with these storage devices and processors are memory devices and data transfer channels, such as data buses, that are all needed for processing the requests. The system further includes some form of interconnect facility through which the clients submit data requests to the processors. This may be a network capable of supporting general purpose communications among clients, processors and other devices, or it may consist of more specialized interconnect facilities such as direct connections. Within one system, there may be many instances of each kind of device and facility. These are all resources of the system; however, they need not all be owned exclusively by the storage system. For example, the processors and memory buses might be involved in other computational tasks that are not part of handling storage requests from the clients.
One request from a client to the system generally does not require exclusive use of all resources. The system is designed therefore to handle many requests from many clients concurrently by scheduling stages in the processing of requests concurrently, such as disk arm motion and data transfer. One of the system's functions for achieving concurrency is queuing, by which the stages of processing for one request can be delayed when other requests are occupying required resources.
Storage service providers often enter into Service Level Agreements (“SLAs”) with data owners, whereby each SLA typically specifies a maximum average response time, i.e. an RTSLA, for requests made by the data owner to write and/or read data to and/or from the SSPs storage facility. When servicing requests from (N) multiple data owners under (N) SLAs, the SSP must allocate system resources such that RT(j)SLA, for each value of (j), is satisfied, where (j) is greater than or equal to 1 and less than or equal to (N).
Although the data objects used by different clients will generally be separate, the storage system resources involved in accessing those data objects will often overlap. These resources may include any of the components described above, such as storage devices, processors, memory, buses, and interconnect. One client's access to data can suffer performance degradation when another client consumes too much of one or more resources. If this competition for resources is not controlled, may be difficult to meet the response times specified in the (N) SLAs. Even if each RT(j)SLA is satisfied, permitting each of the (N) applications to consume arbitrary levels of system resources will not likely generate the maximum revenue for the storage system provider.
Various mechanisms are known in the art to allocate system resources amongst multiple storage system clients. What is needed, however, is a data storage system configured to both satisfy the contractual obligations of the storage system provider, and provide system resources in a way that maximizes the revenue to the storage system provider.